Egypt's military-backed interim presidency has moved to implement a speedy transition to civilian rule, appointing the economist Hazem el-Beblawi as prime minister and the internationally known opposition leader, Mohamed ElBaradei, as vice-president.

In a tense atmosphere after the killing of 55 supporters of the deposed president, Mohamed Morsi, and threats of fresh mass protests by his supporters, the army also warned against political "manoeuvring" at a time of instability and anxiety – apparently to forestall more squabbling about other cabinet posts.

General Abdel-Fatah al-Sisi, the defence minister and armed forces commander who ousted Morsi last week, said in a statement broadcast on state TV that the military was determined to tackle the challenges facing Egypt in "these difficult circumstances". Sisi's message was also a greeting to Egyptians on the occasion of the Muslim Ramadan holiday, which begins on Wednesday.

Beblawi, a respected former finance minister, will lead a technocratic government whose other members have yet to be announced. Crucially, it looks unlikely to include Islamists. The Muslim Brotherhood is holding out for Morsi's restoration, which does not now seem likely.

ElBaradei, a Nobel peace prize laureate, was on the verge of being named prime minister last week, but at the last minute that was blocked by the Salafi Nour party. His role is a fillip for liberals.

Moves towards political stability were swiftly rewarded. Saudi Arabia said it had approved a $5bn (£3.4bn) aid package to Egypt and the UAE agreed to grant it $1bn and lend another $2bn. Both the conservative Gulf monarchies were hostile to Morsi and the Brotherhood.

Earlier on Tuesday, the interim government announced plans for elections and drew up an interim constitution that gave full executive and legislative plans to the interim president, Adly Mansour. The charter was criticised by the Tamarod campaign, the grassroots movement that brought millions to the streets against Morsi in recent weeks. Last night it was also rejected by the liberal National Salvation Front, of which ElBaradei is a member.

But it was welcomed by the US, which had previously expressed concerns about Morsi's removal, but praised the way that Egyptian officials had now "laid out a plan for the path forward".

Morsi supporters were still gathering near the scene of Monday's killings, described as a massacre by the Brotherhood, but defended by the army and a uniformly uncritical state media as a response to a "terrorist" attack.

At the Rabaa Adawiya sit-in, the ground zero of the Islamist presence in east Cairo, the crowds were more sombre than agitated. Mourners left rings of stones where their friends had died, and only a few chanted insults at the soldiers guarding the barbed-wire fence that blocked one entrance to the site.

"It was criminal, it was treason," said Mahmoud Mohamed, a Salafi from Minya, of Monday's massacre. "But protesters are righteous people. We don't know violence. We will only resist with peaceful chanting."

Sherif Mohamed, a teacher from Cairo, said: "The army is trying to falsify the news, cover up their actions. But we are fearless, we are determined. We will continue to stand here in support of legitimacy."

At Cairo's Zeinhom morgue, where many of those killed were taken, mourners were still waiting for their friends' bodies to be released – many still coming to terms with the horror of what happened. "It was barbaric," said Mohamed Abu Sayed, a lecturer at Al-Azhar University, who was waiting for the body of his friend, Mohamed Abdel Rahman. "It was a black day in the history of Egypt's army." Abu Sayed called for Islamists to continue their peaceful resistance in response.

Amnesty International said its inquiries suggested the use of disproportionate force by the security forces. "Many of those killed and injured had been shot in the head and upper body with shotgun pellets and live ammunition," it added.

Morsi, who is now under house arrest, won last year's election by a narrow majority against an old-regime candidate. The president's supporters say he was deposed by a military coup. Opponents call his removal by the military a continuation of the 2011 revolution. The Brotherhood called the appointment of Beblawi and ElBaradei "a deal with putschists".